Evolution of the C-H aliphatic component of the interstellar matter under cosmic ray irradiation. The values of σd were determined from a-C:H 1 data with α = 1.0 (see Fig. 5). The black curve corresponds to an exponential decrease without residual hydrogen content taking into account (ρ/ρi = exp(−Rd,CR·t)). The red solid (fit 1) and dashed (fit 2) curves are obtained when the asymptotic values ρf/ρi ≠ 0 are considered. Values of ρf/ρi are determined from a-C:H 1 data and two different fits (see Fig. 6 and text for details). The dotted vertical line corresponds to a typical cloud time scale t = 3 × 107 yr. (McKee 1989; Draine 1990; Jones et al. 1994).
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